1. Field of the Invention
This invention relates to electrical energy measuring circuits used to measure electrical energy consumption by computing the product, and the time integral of the product, of two parameters of an electrical energy quantity. More specifically, this invention relates to those electrical energy measuring circuits which utilize a mutual inductance current sensing transducer capable of sensing widely varying amplitudes of the current. The mutual inductance current sensing transducer is used to produce one of the parameters of the electrical energy quantity to be measured.
2. Description of the Prior Art
For measuring the electrical energy consumption of separate electrical energy users the electromechanical rotating disc-type of watthour meter continues to be used almost exclusively. This widespread use is justified because of the meter's high degree of accuracy and reliability during many years of service under widely varying ambient operating conditions. It is also justified because of the meter's low cost, and the ease with which it can be installed.
Solid state watthour meters are being investigated for the same reasons identified above. However, it is also expected that solid state watthour meters will be capable of performing additional functions, such as load control, measurement of various parameters of electrical energy consumption, time-of-day metering, and communicating with remote meter reading equipment. Accordingly, much investigation is being carried out in the area of solid state watthour meters.
One characteristic feature of solid state meters is the need for low level input signals. At typical customer locations line voltages of either 120 or 240 volts define the voltage component of the electrical energy quantity to be measured. Standard potential transformer arrangements can provide practical voltage sensing transducers. On the other hand, load currents which define the current component of the electrical energy quantity to be measured vary considerably. A substantially linear response is desired in the range from 1/2 ampere to 200 amperes. Current transformers producing low level output signals from the aforementioned widely varying input signals are of substantial size and cost.
For the reasons cited above, it is desirable to provide current sensing transducers which are accurate, inexpensive, of a convenient size, and are capable of producing the low level signals required by AC electrical energy measuring circuits. A transducer meeting these requirements is disclosed in U.S. patent application Ser. No. 923,619, filed July 11, 1978, for a "Mutual Inductance Current Transducer For AC Electrical Energy Meters". This current sensing transducer provides an analog signal proportional to the time derivative of the line current. In light of this development, it has become advantageous to provide a solid state electrical energy measuring circuit capable of utilizing this time derivative type of input signal.
An invention which has recognized the advantages of using a solid state meter which can operate using a time derivative input signal is disclosed in U.S. patent application Ser. No. 923,530, filed July 11, 1978, and assigned to the assignee of this invention. An AC electrical energy measuring circuit is disclosed for converting an analog input signal, which is proportional to the time derivative of the current component, into a pulse width modulated signal (PWM signal). This PWM signal is then amplitude modulated by a signal proportional to the voltage component to produce a variable amplitude pulse width modulated signal having an average value equal to the voltage component times the current component.
Another example is U.S. patent application Ser. No. 919,874, filed June 26, 1978, and assigned to the assignee of this invention. An electrical energy meter is disclosed wherein the voltage component of an electrical energy quantity to be measured is converted by electrical circuit techniques to a signal proportional to the time integral of the voltage component. The time integral voltage signal is compared to incremental reference levels. Each instance that a referenced level is reached, the instantaneous magnitude of the current component is sampled and converted to digital signals. These digital signals are summed to produce an output signal corresponding to a measure of electrical energy in watt-hours.
The present invention is for a solid state multimeter which utilizes a time derivative input signal to compute the electrical energy consumption.